In recent years, fiber optic technology has formed the basis for different types of sensors, such as microphones, pressure sensors, strain sensors, and others. Optical fiber sensors can use interferometry or intensity modulation, of which intensity modulated fiber optic sensors are simpler and less expensive.
Intensity modulated fiber optic sensors have the advantages of being highly accurate, resistant to electromagnetic interference (“EMI”), capable of being multiplexed, capable of long-distance sensing applications, physically robust, and physically simple when compared to alternative measurement approaches for the physical effects of interest. Since fiber optics use light rather than electricity, a fiber optic sensor is generally insensitive to EMI and is therefore more efficient in an environment that has a large amount of electromagnetic energy. As a result, fiber optic sensors can be located adjacent to or attached to circuits that generate large electromagnetic fields or in areas with high potential for EMI, without negative effects to either the measurement or the measuring equipment.
Geomagnetically-induced currents (“GICs”) are produced by solar storms, which emanate from the sun as coronal mass ejections (“CMEs”), can disturb the Earth's geomagnetic field over wide geographic regions. See, e.g., North American Electric Reliability Corporation, Geo-Magnetic Disturbances (GMD): Monitoring, Mitigation, and Next Steps (2011). The disturbance in the Earth's geomagnetic field caused by a CME (or in the case of an electromagnetic pulse, or “EMP”, by a man-made mechanism) can create currents to flow in the ground which flow into any connected conducting structure, such as the grounded neutral of a power transformer. GICs are expressed as direct currents (“DC”) or quasi-DC currents (with frequencies below 1 Hz), although the precise variation in frequency is dependent upon the time variation of the electric field. Because the cross-sectional area of the single-turn loop represented by the power line and a ground return is large, the current produced can be large (i.e. −100 amps or more). Once GICs or EMPs enter into the bulk power system's transmission and generation facilities, they flow along available conducting paths such as electric transmission lines, metallic pipelines, telecommunications cables, and railways.
When GICs or EMP currents enter power system equipment and facilities they may produce harmful short-term and long-term impacts to power systems, including major increases in system reactive requirements, potential for permanent equipment damage, and the disruption of interconnected system operation. In electric networks, GICs can saturate and may damage some equipment, which can be difficult and expensive to replace in a timely manner. Without adequate steps to mitigate their effects, these currents can disrupt the normal operation of the electric power system, damage equipment, and pose a substantial risk to system reliability. Studies of the potential impact of substantial GMD have found that a major event could leave 150 million Americans without power for an extended period, place more than 350 Bulk-Power System transformers at risk of permanent damage, and cause economic damages as high as $2 trillion. See Oak Ridge National Laboratory, Geomagnetic Storms and Their Impacts on the U.S. Power Grid, Meta-R-319 at 4-14. A severe geomagnetic storm in 1989 affected the Hydro-Quebec power grid resulting in a cascading failure of the system, leaving over six million people without power for several hours and causing significant economic loss. Other severe geomagnetic disturbances have also been documented, including the 1859 Carrington Event that is estimated to have been substantially greater than the 1989 Hydro-Quebec event. See Pacific Northwest National Laboratory, Geomagnetic Storms and Long-Term Impacts on Power Systems (2011).
Given the potential impact of a significant GIC or EMP event, there has been increasing emphasis upon the need for approaches that can accurately identify and measure GICs and EMPs in order that system operators may take steps to mitigate their potential effects upon the reliable operation of electrical networks. See, e.g., Federal Energy Regulatory Commission, Order No. 779, Reliability Standards for Geomagnetic Disturbances (2013).
Traditional approaches for measuring current in electric power systems cannot readily measure the low frequency (sub-1 Hz) electric phenomena associated with these events. This is related primarily to the fact that electric power system measuring and monitoring devices are intended to measure the alternating currents (“AC”) typically associated with power system operation and requires the presence of an AC current to generate its measurement. Due to the need for an AC current to perform measurement, traditional current transformers (“CTs”), a prevalent current measuring device, cannot readily measure direct currents. A resistive shunt can be used, but these devices have a strong temperature dependency due to their construction as a resistor, have a relatively short useful life, and require regular recalibration in the field in order to maintain an accurate level of measurement which increases the long-term operating costs associated with these devices. Resistive shunts also increase in size, weight, and complexity as the rated current measurement increases. These devices require a direct electrical connection, which can create a safety risk both to the equipment itself and to personnel in the event of a fault. More recently, interferometry based optical measurement devices have been used, but these devices tend to be very expensive in addition to having significant temperature dependency issues. These devices also use lasers as a light source, have complicated associated light-measuring techniques, and require regular recalibration, all of which raise the complexity and cost associated with this technique.
Accurate information is critical regarding the presence of GIC or EMP-induced currents in the operation of electrical power generation, transmission, and distribution systems. What is needed is a system that can accurately measure DC and quasi-DC currents, is di-electric in nature, is robust enough for field use, and is relatively non-complex. Intensity modulated fiber optic sensors are capable of accurately measuring DC and quasi-DC currents (i.e. −very low frequency) associated with GMDs and EMPs. This information can be used to determine the presence and magnitude of potentially harmful currents being introduced into key elements of the electric grid through the grounded neutral of a power transformer or a system of power transformers and into other components of the electrical power system, such as transmission lines. Fiber optic sensors also have inherent advantages in terms of accuracy, EMI sensitivity, safety, size, and robustness compared with existing measurement devices and systems that make them a preferable approach to measure such currents.
The system disclosed presents an approach by which sensors can measure multiple instances of currents associated with GMDs or EMPs, as well as other information regarding the physical operation of key assets—such as other electromagnetic phenomena (voltage, current, electric or magnetic fields), pressure, strain, displacement, acceleration, temperature, or other physical phenomena. The system thereby generates the information necessary for system operators to monitor key system assets for potentially harmful currents and other physical phenomena, including those that may be indicative of a system disturbance, and take appropriate action in response.